The process of spraying products with a solid powder coating involves preparing the powder coating in finely ground form and spraying it onto the parts in a manner similar to liquid paint. Conventionally, but not necessarily, an electrostatic charge is applied to the powder as it is sprayed toward an object to be coated. The object to be coated is maintained at an electrostatic potential different from that of the charged powder particles so that the particulate powder material is attracted to the article and deposited thereon with improved efficiency and coverage. The electrostatic charge maintains the powder on the product for a sufficient time period to permit the powder to be heated so that it melts, and when subsequently cooled the powder is firmly attached to the target substrate.
In industrial finishing applications for particulate powder materials, it is often necessary to change the color of the powder coating material from time to time. One approach in the prior art to convert from one color to another has been to employ multiple spray guns each connected to separate supply lines which deliver a different color of particulate powder material to each gun. These systems include controls upstream from the powder spray guns to control the flow of powder material so that when one color is desired the supply line carrying such color is opened and the other supply lines are closed. Although this system is effective in quickly changing from one color to another, it requires the use of a number of spray guns and a control system for opening and closing the separate supply lines to each spray gun. As a result, such systems are relatively costly to purchase and maintain.
Prior art systems for changing from one color of particulate powder material to another have also been proposed which incorporate a single spray gun connected to a multi-ported manifold. Supply lines from sources of a number of different colors of powder material are connected to the manifold and then via the manifold to one spray gun. See, for example, U.S. Pat. No. 4,248,379 which is assigned to the same assignee as this invention. In changing from one color of particulate powder material to another, a pressurized flow of air is employed to purge the manifold, the feed line leading from the manifold to the spray gun, and the interior surfaces of the spray gun which come into contact with the powder. This purging flow of air is directed through the spray gun while maintaining it assembled, and in position within a powder spray booth or other enclosed powder spraying apparatus.
One limitation of prior art systems of the type disclosed in U.S. Pat. No. 4,248,379, is that it may be difficult to completely clean each part of the spray gun which contacts the powder without disassembling such parts. This is particularly true where there are bends or corners in the powder path formed by parts of the spray gun within which the finely ground powder material can lodge. Additionally, no provision is made in such a powder cleaning system to remove powder coating material from the outside of the spray gun. If any remnants of a first color of particulate powder material remain on the interior or exterior surfaces of the gun when a particulate powder material of a second color is discharged therethrough, these colors can combine to produce an unacceptable coating when the production run is first resumed.